This invention relates to a improved method of heat exchange between a first fluid inside a heat exchange tube and a second fluid on the outside of the heat exchange tube.
When heat is transferred from one fluid to another fluid through a solid wall (e.g., the wall of the tube) the magnitude of heat transfer depends on (a) surface area, (b) the value of heat transfer coefficient at the inside and outside surface of the tube, (c) the thermal conductivity of the tube wall material, (d) the wall thickness, and (e) temperatures of the participating fluid. Therefore, for given fluids and their temperatures and for a given tube material and size, the magnitude of heat transfer can be significantly increased by augmenting heat transfer at the inside or outside or on both sides of the tube wall. Such augmentation is defined as the improvement of the convective heat transfer coefficient. Therefore, the method of this invention is useful in economizing design and manufacture of a large variety of heat exchange equipment by reducing the heat transfer surface for given magnitude of heat transfer. However the method of this invention is simple enough to implement in new or existing heat transfer equipment and economical enough to justify its cost compared with the benefits it can derive.
A turbulator can most effectively improve heat transfer if it is capable of generating a high level of turbulence in the boundary layer very close to the wall surface. The turbulators that are presently available in the market do not effectively generate such high levels of turbulence close to the wall surface through which heat transfer actually takes place. For example, the turbulators disclosed in Smick, U.S. Pat. No. 4,044,796, and Burke, U.S. Pat. No. 4,412,558, are both made of a metal strip of suitable width formed in a zigzag shape. When such turbulators are placed inside a tube, a portion of the tube opening is blocked and, therefore, the fluid has to flow around the turbulator strip. As the fluid passes over the strip, turbulence is generated primarily in the bulk fluid due to flow separation. The turbulence thus created eventually transmits into the boundary layer where the viscosity of the fluid dampens down the intensity of the turbulence, meaning reduced effectiveness in the augmentation of heat transfer. Moreover, the zigzag strip is limited for the tube's internal augmentation and it is not suitable for liquids, especially for viscous liquids due to the high pressure drop and reduced effectiveness.
Another kind of turbulator presently available in the market is known as a spinner turbulator, comprises a twisted metal strip. It is inserted inside a tube. It is used for both liquid and gas. As the flow progresses through the tube, the fluid gains spinning motion as it follows the contoured path of the twisted strip and thereby exerts centrifugal force on the tube wall and partially distorts the boundary layer. Also the combined effect of spinning and translatory motion of the fluid increases the turbulence level in the bulk fluid which eventually transmits into the boundary layer, but at a reduced intensity due to viscous effect of the fluid. Again, this type of turbulator is limited for insertion inside a tube.
The boundary layer turbulators used in the present invention are effective heat transfer augmenters Their primary function, unlike the other turbulators, is to generate a very high level of turbulence right in the boundary layer very close to the wall surface where heat transfer actually takes place. The turbulence thus generated propagates from the boundary layer to bulk fluid.
The comprised turbulator consists of a wire or ribbon coiled in the form of a helical spring placed inside the tube to extend the entire length of the tube. The outside diameter of the coil is equal to, or slightly less than, the inside diameter of the tube (in the case of a round tube) so that the wire or ribbon is placed against the wall surface or very close to it. When the flow of fluid trips over the wire, a high level of turbulence is generated in the boundary layer close to the wall surface, thus augmenting the heat transfer. Similarly, a helical coil can be put on the outside surface of the tube for augmentation of heat transfer at the outer surface when a fluid flows over the tube in the direction of the tube axis. Such helical coil-like turbulator is called a boundary layer turbulator. According to the method of this invention, the heat transfer coefficient inside the tube can be further increased when the boundary layer turbulator used inside the tube in conjunction with a spinner turbulator. Under this situation, centrifugal force due to circulatory motion of the bulk fluid helps to generate even higher levels of turbulence in the boundary layer extremely close to the wall and thus resulting in even higher levels of heat transfer augmentation. The application of such augmentation is only limited for the inside surface of the tube.
The ultimate object of the method of this invention is to apply a compound turbulator in an improved method of heat transfer in which the heat transfer surface of the heat exchange equipment can be significantly reduced. Thus, a significant savings can be obtained in fabricating this equipment. Such heat exchange equipment can exchange heat between any two fluids such as between a liquid and a liquid, or a gas and a liquid. The first fluid inside the heat exchange tube is highly turbulated, and thus substantially homogenous.